1. Field of the Invention
The invention relates to bulk material analyzers, and more particularly, to a method and apparatus for determining the compositional elements of bulk materials utilizing one or more neutron sources and one or more radiation detectors.
2. Description of the Related Art
Atomic research has led to many uses of the neutron, including material analysis. When an atom absorbs a neutron, it increases in atomic weight, but at that moment, the chemical properties of the atom do not change, thus forming a new isotope of the same element. When a neutron is absorbed, the absorbing atom emits one or more gamma rays, the number and energies of which are unique to that element. The new isotope may be unstable and seek stability by emitting one or more forms of radiation over a period of time, which may also result in the atom changing to a different element. Every radioactive isotope has a characteristic half-life as it decays to a stable state. An element that has absorbed a neutron can be identified by either the absorption gamma rays that it emits or by the decay-radiation it emits. The latter is normally referred to as neutron activation analysis and the former is often called prompt-gamma, neutron activation analysis (PGNAA).
Since the neutron was discovered, and especially during the period of the 1940s through the 1960s, both prompt and delayed radiation emissions from neutron absorption have been carefully catalogued at laboratories and universities around the world. PGNAA was applied to coal analysis at the U.S. Bureau of Mines in West Virginia (Stuart and Hall, “On Line Monitoring of Major Ash Elements in Coal Conversion Process,” Reprint 789671, October 1978, 13th Intersociety Energy Conversion Engineering Conference, Society of Automotive Engineer, Inc. Warrendale, Pa., pp 586–591) and through research sponsored by EPRI during the 1970s and 1980s. Commercial PGNAA analyzers were introduced during the 1970s and 1980s.
Non-homogeneous industrial materials, such as coal, cement ore, bauxite, kaolin, etc. are ideal candidates for PGNAA. The traditional method of determining average elemental composition of such ores includes taking a representative sample to a laboratory, and most material-analysis techniques used in laboratories assume a homogeneous sample and perform one or more surface measurements. However, obtaining a representative sample of a non-homogeneous bulk material is both expensive and time-consuming. A few grams of sample analyzed in a laboratory are estimated to be representative of the total, which may be hundreds of tons of the bulk material. This large discrepancy between the size of the sample and the size of the actual bulk material is a major source of error in such a measurement. In addition, the inherent delay between sampling the material and obtaining a final measurement in this system is typically on the order of hours, which prevents real-time process control that may be desirable due to changes in material composition.
PGNAA can inherently measure material composition throughout a relatively large volume of material because neutrons penetrate matter to a great depth and the resulting prompt gamma rays are of energies high enough to permit them to escape from a substantial depth within the material. When the bulk material is bombarded with the neutron radiation, different characteristic gamma-ray energy spectra are produced from different elements in the bulk material. By processing detected signals indicative of gamma ray energies, a measurement can be made regarding the elemental content of the bulk material. Directing a PGNAA analyzer at a stream of industrial material can allow the full stream to be measured in real time, thus eliminating sampling error, allowing measurement of instantaneous variations in material, and allowing on-line, closed-loop control of material processes based on material composition.
Commercially available PGNAA analyzers are generally too large (typically weighing several tons and occupying more than 20 cubic meters of space) and too expensive (generally in excess of $300,000) to be used in many applications where they could be beneficial. A significant reduction in size and price of an analyzer while maintaining or improving performance as compared with currently available analyzers would therefore be beneficial in the art.